1. Field Of The Invention
The present invention relates generally to a method for reducing pollution and increasing waste heat recovery in power generation systems and combustion methods, and for an arrangement of apparatus for carrying out the method. More particularly, the present invention relates to both reduced pollution and increased energy recovery from the flue gas by operating with flue gas recirculation at increased pressures, with direct contact heat exchange, and with higher oxygen concentrations in influents. An embodiment of this invention includes a regenerative Brayton cycle using a slurry fuel. This invention has useful applications in combustion of biomass, coal slurries, sewage, and garbage as well as other energy conversion methods such as air conditioning.
2. Brief Review of Prior Art
Historically, the energy of water vapors in flue gases has been regarded as not recoverable in a form of sufficiently high quality to warrant development of methods based around this concept.
U.S. patent 4,898,107 recognizes the benefits of burning liquid or solid wastes at increased pressures to recover more latent heat from the water vapor in the flue gas. However, U.S. Pat. No. 4,898,207 is based on configurations operating at relatively low temperatures and relying on alkali addition to the combustion method as a primary means of pollution control.
U.S. Pat. Nos. 4,714,032, 4,377,066, and 4,380,960 refer to combustion at high pressure conditions and in the presence of excess water. These patents are very specific with respect to addition of alkali to the combustion feed or the nature of the fuels. The embodiments of this invention can improve the performance of these patents as well as U.S. Pat. No. 4,898,107. However, the embodiments of this invention are not limited to the embodiments of said patents.
Furthermore, U.S. Pat. No. 2,677,234 is based on the pressurized combustion of water containing fuels; however, methods of the present invention are more advanced in conserving energy (the availability of energy to be convened into work). U.S. Pat. No. 5,067,317 is based on the pressurized combustion of slurry fuels and condenses water in the flue gas to be used in creating the slurry fuels. The present invention recirculates water in a different manner than these patents with specific advantages associated with allowing lower quality (lower temperature) heat to be used to evaporate water contacting influent gases.
U.S. Pat. No. 3,328,957 describes an underwater propulsion system in which water can be recycled in the system. The present invention recirculates water in a different manner with specific advantages associated with allowing lower quality (lower temperature) heat to evaporate water in influents.
U.S. Pat. No. 4,238,925 describes a combustion method in which liquid oxygen is used as an oxidant for the combustion of fuel and utilizes flue gas recirculation and heat exchange from flue gas to influents prior to combustion. Methods and associated advantages described in U.S. Pat. No. 4,238,925 are not similar to those claimed by the present invention.
U.S. Pat. No. 5,069,692 describes a system utilizing an air separation means to provide for on board (aircraft) generation of inert gas and oxygen which is available to the crew and for combustion. U.S. Pat. No. 5,069,692 discloses methods for air separation with membranes; however, the combustion methods are not similar to those of the present invention.
U.S. Pat. No. 4,909,029 describes fluidized bed firing equipment at a pressurized environment and operating under a turbocharge. The embodiments of the present invention are not similar to the mechanical configurations of the said patent.
U.S. Pat. No. 4,829,938 describes a novel method on how to operate a denitrification unit. The present invention is not specific to configurations of heat exchangers, economizers, and denitrification units. Such configurations as described in U.S. Pat. No. 4,829,938 can be directly applied to the present invention based on the quality of heat available at specific locations.
U.S. Pat. 4,799,941 is based on increasing the production of low grade heat by humidifying entering oxidants and then extracting the latent heat from resulting flue gasses. The author points out that this method is particularly advantageous when burning wood chips or other organic matter. The present invention produces higher grade heat by a different method based on water condensation under pressure.
U.S. Pat. No. 4,542,621 refers to a method of scrubbing sulfur containing flue gases. This method relies on mixing previously scrubbed and heated flue gases with more recently scrubbed flue gases to prevent misting from occurring when the flue gases are released to the environment. The present invention is enhanced over this patent in that the total volume of gases released is substantially reduced and reheating the flue gas is not necessary, especially if a demister is used in the exhaust stack.
U.S. Pat. No. 4,414,923 describes a heat recovery boiler for high pressure gas. Although such embodiments may be useful with the present invention, the said embodiment is not necessary for the successful operation of this invention.
Oxygen enriching embodiments are described in U.S. Pat. Nos. 4,894,068, 4,696,686, 4,599,157, and 4,542,010; however, none of these embodiments are associated with combustion methods. U.S. Pat. No. 4,883,023 describes a turbocharger mechanism operatively associated with oxygen enriching mechanisms; however, the methods, embodiments, and purpose of U.S. Pat. No. 4,883,023 are not similar to those of the present invention.
Some of the novelty of the present invention resides on the method of direct contact heat exchange. Whereas conventional closed heat exchangers rely on a solid wall to transfer heat between two streams, direct contact heat exchange occurs between two streams in direct contact. Demonstrated systems include water-air in cooling towers, clinker-air in cement kilns, vapor-liquid in distillation columns, and molten nitrate salt-air in a solar flux receiver (see Direct-Contact Heat Transfer by Frank Kreith and R. F. Boehm). For most of these applications, direct contact heat exchange has become very popular. In fact, cited advantages of direct contact heat exchange include reduced capital costs, eliminated fouling of solid heat exchange surfaces, eliminated thermal stress and corrosion problems, increased heat transfer coefficients, and readily increased surface areas for heat transfer. The present invention uses direct contact heat exchange in new and advantageous methods.
The present invention consists of a method for burning slurry, liquid, or gaseous fuels at elevated pressures allows lower quality fuels to yield more energy and higher quality energy due to increased dew point temperatures in flue gases and high temperature heat exchange. The combination of elevated pressures and oxygen rich oxidant allows increased waste heat recovery, higher quality heat recovery, and substantially reduced air pollution. Turbochargers operated by flue gas and pressurized air, force the oxygen into the combustion chamber at increased pressures. Oxygen permeable membranes lower levels of nitrogen and reduce pollution by enhancing the stripping of pollutants from the flue gas and by the absolute reduction of flue gas exhaust. Latent heat recovery and water dilution of combustion mixtures are important aspects of the embodiments of this invention; both are enhanced by the vaporization of influent liquids while contacting influent gasses. High temperature heat exchange is facilitated by direct contact heat exchange means. Substantially reduced pollution levels allow operation in residential and downtown areas where cogenerated heat and coolant can be utilized. This method is particularly useful for converting garbage or sewage into electricity and for increased biomass combustion efficiency; however, advantages of this method can also be realized for most conventional fuels.